The present invention relates to an apparatus and a system for generation of an intentional sidelobe in adaptive antenna systems for mobile telephony, particularly for systems utilizing PDC standard.
In cellular mobile telephony systems of today the base stations utilize antennas for omnidirectional radiation or sectorized radiation (each sector typically having 120 degrees of coverage). The antenna patterns then will cover the entire cell and no knowledge of the particular position of the mobile stations is normally used. For increasing range and capacity, beam shaping solutions like antenna arrays may be used. The narrow beams of such an array antenna may be utilized to improve the link-budget in both uplink and downlink and to reduce interference in both uplink and downlink. In Time Division Multiple Access systems (TDMA) it is sufficient to transmit signal, for teach user, during a current time slot within a narrow sector in a direction towards the mobile station. This is generally illustrated in FIG. 1.
Users of the system will have channels, a specific time slot on a specific frequency allocated in such a way that mutual interference between different users should be minimized. Consequently, the idea is to avoid spreading power in directions, which the present communication is unable to use, i.e. thereby minimizing interference within the system. The level of interference affects the channel re-use pattern in the cellular system. Without changing the cellular planning the interference level will according to such a method be decreased by utilizing beam shaping with narrow beams based on knowledge of positions of the respective mobile stations communicating with a certain base station. This lowering of interference may be used for either accomplishing increased capacity in the system (i.e. a tighter re-use of frequency channels) or for accomplishing improved quality in the current communication links (i.e. increased data or voice quality for the final user). There are found a number of documents disclosing different ways for obtaining radiated power narrowly directed toward a station currently receiving communication over a certain time slot in a TDMA system. For instance international applications WO94/11956 and WO96/29838 are disclosing systems adopted to match a base station""s radiated field in one direction towards the particular current mobile station communicated during a particular time slot.
In the Japanese PDC system for mobile telephony each mobile station can have two or more antennas. During the last millisecond before reception of an allocated time slot, see FIG. 1, the mobile telephone measures power received in its respective antennas. The antenna showing best performance then will be used during the time slot containing information for the particular mobile. This diversity function provides a complicating factor for a concept using adaptive antennas for the PDC system.
In order to have this antenna selection function in such a system it is required, according to the PDC standard, that the power measured by the mobile station not to be more than 8 dB below maximal power from the base station. If no particular measures are taken, a narrow signal beam used towards a current mobile station results in that a mobile station, which is to be active for reception in the next time slot, runs the risk of having a deteriorated or totally spoiled diversity function due to not detecting enough power at the end of the previous time slot.
In order to generate more narrow beams giving better directivity for the transmitted power an antenna array will normally be used. However, to be able to generate an arbitrary antenna pattern, it is necessary that the different radiating elements in the antenna array being fed with coherent signals, i.e. with small amplitude and phase errors.
A well-known method to reduce the coherency demand is to utilize a so called Butler matrix, which in an efficient manner performs an analog beam shaping However, the beams formed will in part be statically decided by the combination network and in part have a sidelobe level typically lower than xe2x88x9213 dB relative to the main beam. What the Butler matrix essentially does is to direct the main beam to a certain direction. Each beam port of the Butler matrix corresponds to a specific controlled directive angle. By combining a number of these fixed beams the same possibility of arbitrary beam shaping as in a case of not having a Butler matrix exists. However this implies on the other hand that the demands of coherency will be strong.
The demand of equality between signal paths in amplitude will be of the order 1 dB and of the order 20 degrees in phase. This is valid independently of having a system using a Butler matrix or not.
A solution to the general problem should be to form an antenna pattern which enables the mobile station in a current time slot to receive full power simultaneous as the mobile station in the following time slot also receives a signal strong enough for the diversity function to operate.
The general inventive idea for an adaptive antenna device which solves the present problem is based on an array combination having realized low coherency demands. This may be, for instance, achieved by means of a Butler matrix. However, there should also be a possibility to generate additional sidelobe power which does not show sidelobe levels of typically xe2x88x9213 dB or less relative to the main beam normally obtained, but maintaining a sidelobe in a desired direction at a preferred level of about xe2x88x928 dB in accordance with the PDC standard for its diversity function to operate properly.
The principle generally implies that fixed beams of a Butler Matrix may be utilized. In the beam having the best performance for the present downlink a signal carrying information will be transmitted. In another beam, which in a corresponding manner has the best performance in regard to the next mobile station to be communicated in a next time slot, a signal will be sent which has a low cross correlation relative to the present signal carrying information.
Utilizing a fact that both signals then are xe2x80x98essentially orthogonalxe2x80x99, the average power in the direction to the next mobile station to be communicated will be given by the sum of the average power of the two signals after amplification by the respective antenna pattern. By the expression xe2x80x98essentially orthogonalxe2x80x99 is indicated that the cross correlation between the signals should be low enough so the sum will be a power addition but not a coherent addition.
This implies that in this case there does not exist any demands on a mutual coherency of the transmitted signals for the main beam and the created intended sidelobe, respectively.